add duplicate selection shortcut to hotkeys as part of editing command (#6843)

## 📝 Summary

Fixes #6338 


## 🔍 Description of Changes

Adds an additional keyboard hotkey (that can be modified) for the
duplicate selection shortcut and is visible from user settings under
Editing (see below for screenshot of the change). The name is modified
from copy line down since it technically copies the entire selection and
not just a specific line on execution.

<img width="942" height="125" alt="Screenshot_20251020_121742"
src="https://github.com/user-attachments/assets/668687a1-7109-454a-855f-437ebc6c599b"
/>

## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

---------

Co-authored-by: Myles Scolnick <[email protected]>

Author: minsun-ss

frontend/src/core/hotkeys/hotkeys.ts

@@ -239,6 +239,18 @@ const DEFAULT_HOT_KEY = {
     group: "Editing",
     key: "F2",
   },
+  "cell.copyLineUp": {
+    name: "Copy line(s) up",
+    group: "Editing",
+    key: "Alt-Shift-ArrowUp",
+    editable: false,
+  },
+  "cell.copyLineDown": {
+    name: "Copy line(s) down",
+    group: "Editing",
+    key: "Alt-Shift-ArrowDown",
+    editable: false,
+  },
 
   // Markdown
   "markdown.bold": {